1. Field of the Invention
This invention relates to telephone subscribers' sets and, more particularly, to an automatic telephone subscriber's set which is able to generate calling signals in the form of decimal code pulse trains and multifrequency code a.c. signals according to CCITT Recommendation number Q 23.
2. Description of the Prior Art
Subscribers' sets connected to automatic telephone exchanges include a device by which a user can transmit to the exchange equipment the information necessary to set up a required connection. The device may include a dial or a series of twelve push-buttons forming a keyboard. Conventional automatic telephone exchange equipment is operated by decimal code pulse trains. Some recent telephone exchanges have multifrequency code receivers enabling the called number to be transmitted not as trains of pulses but as multifrequency combinations in a code "p out of n".
Push-button telephone sets are available for use with both decimal code calling signal operated exchanges and multifrequency code calling signal operated exchanges. These telephone sets comprise a pulse generator means for generating decimal pulse trains and a frequency generator means for generating multifrequency calling signals. Switching means are provided for selectively connecting either the pulse generator means or the frequency generator means to a subscriber's line.
The normal access procedure for automatic telephone subscriber is for the subscriber to call the computer unit and, once connected thereto, to transmit data and control signals. Accordingly, when the subscriber is connected to an exchange operated by decimal code calling signals, he has to send direct current pulse trains to the switching network for implementing connection to the computer unit and then to transmit multifrequency code alternating current signals.
To change over from pulse transmission during dialling to frequency transmission after trunking, there are two possibilities: -- switching the keyboard to one or the other of the two transmitters both connected to the line as taught by British Patent No. 1,298,077 or switching to line the output of one or the other of the two transmitters both connected to the keyboard as taught by British Pat. No. 1,307,266 (which comes to the same thing), in dependence upon the position of a changeover button at the station. This changeover button can be one of the existing push-buttons of the keyboard. However, such push-button must not be one which triggers the pulse transmitter so as to avoid interrupting the call. Such push-buttons are available in the form of keys Nos. 11 and 12, marked * and #, of the telephone keyboard, which are not used for pulse transmission. Thus no special button is required. All that the subscriber who wishes to use his station as a computer terminal has to do is to follow dialling and trunking by actuation of the * or # key before transmitting data to the computer. The disadvantage of this kind of operation is that it is different from what a subscriber having just a push-button set connected to an exchange having multifrequency code receivers has to do, since the latter kind of subscriber does not have to operate an extra key after trunking and before data transmission.